Charcoal Clamps (Charcoal 2)

Charcoal Clamps: The Mound Method

To read about what charcoal is, see The History of Charcoal.

Site prep

Charcoal production begins with preparing a burn site. This may be a trench or a pit, but it can also be a cleared above-ground surface at a safe distance from flammable materials. The charcoal burner had to scrape the flat site of all vegetable matter, including surface roots, which could spread the fire from the burn. Later techniques for this included using draft animals and a scraper blade to drag the area clean; in the Appalachian Mountains, colliers chose saddles between ridges and clear-cut, pulled out stumps, then scraped with a road blade to create the round, flat, clear spaces for their charcoal burns (and many of these clearings, with fire-road access, still serve as convenient camping spots today). Details on their method can be found in the Foxfire series, Vol. 5.

For the mound method, it was imperative that the burn site be stable and completely level. If the ground sloped, the stacked-up dirt covering the mound could slip unevenly and cause cracks or gaps, allowing oxygen into the burn when it was smothered and ruining the batch. Stacking and digging out were also both better done on flat ground than where logs might roll away.

Frequently, artificial flat ground had to be constructed by cut-and-fill digging, if level spots large enough for a burn mound were not available. These “charcoal platforms,” and other above-ground “charcoal clamps” (1) can be found in the archaeological record by the high-temperature burn evidence in the soil and the remnants of charcoal left behind. Depending on the terrain and soil, building a charcoal platform could also require stone retaining walls or bricks, which helps in identifying the original dimensions of the charcoal burn.

Materials

Once the burn platform or cleared site was prepared, the charcoal burner had to collect the material for charring. This could be peat, deadwood, bark, nutshells or other burnable materials, but the overwhelming majority of evidence demonstrates a preference for seasoned oak, often grown in coppices.

First, the living oak was felled, ideally in freezing weather when the sap was below ground. If the burn site was far away, the log might be bucked (cut) into cordwood (short segments of equal lengths, typically between 0.5m to 1.5m) where it had fallen. This cordwood was then stacked, usually under cover, to season (dry out) for 6 months or more. This process allowed the moisture content of the wood to drop from about 60% to about 15%, ensuring a hotter and much more efficient burn. Once dry, the cordwood weighed about half of its original wet weight, making it much easier to transport to the burn site. Of course, if the forest was conveniently nearby to the burn site, then the log would be dragged there first and bucked, stacked and seasoned at the burn site itself.

Immediately prior to constructing the burn pile, the charcoal burner placed a chimney log or constructed a stacked-stick square chimney in the center of the burn site. A chimney log would hold an opening at the center of the pile, and be withdrawn prior to lighting the pile. A stacked chimney would be made of burnable kindling and would help to start the fire inside the pile, while protecting the opening for smoke and steam to vent out of the pile during lighting.

The charcoal burner also could place vent logs (typically thin saplings) on the ground of the cleared site, sometimes propped up on one another, to create air gaps at the bottom of the pile. These allowed fresh air to draft in from below during the lighting of the pile, and helped heat circulate within the pile after smothering. The more circulation of gases inside the pile, the more evenly the entire mound would heat and the more completely all of the wood would char (2).

Finally, the charcoal burner needed to collect green mulch; pine needles and branches, fresh-cut grass, leaves, straw, etc all worked, depending on what was available. These needed to be green enough to keep the upper dirt layer from touching the fuel wood below, protecting the resulting charcoal from getting dirty. If the mulch layer burned away, the dirt above would fall into the pile—so green, fresh mulch worked best, and the mulch layer had to be more than a hands' depth thick.

Stacking



With the chimney in place, and ideally the vent logs laid around it, it was time to stack the fuel wood.

The seasoned cordwood would be piled carefully into a mound shape, normally round or subrectangular, with the chimney at its center. The fuel logs could be laid horizontally, or placed upright on end, and would be built into the pile in layers radiating out from the center. Gaps between the logs were not desirable, so small wood could be inserted between uneven logs to fill the space. Mounds could vary enormously in size, but the larger the mound, the more efficient it was in terms of fuel and maintenance labor. A typical round mound measures between 6m to12m across (20-40 feet diameter) and typically at least 2m tall (3).

Next, the charcoal burner would spread a thick layer of green mulch over the stacked mound—forming a barrier to keep the wood clean. Onto this protective layer, he would pile sand or gritty dirt (if available), then regular dirt, wet mud, and/or clay, very thickly, leaving only the central chimney and the vent hole uncovered. It was critical to get everything thoroughly covered and sealed air-tight. Using loose dirt and sand ensured that the settling, shrinking pile would remain covered and that the dirt would settle with it. Cut sod would not settle as effectively, and was more valuable in other contexts, so this cover was typically made of the local loose dirt.

Only the ends of the vent logs, and the openings they protected, would be left uncovered by dirt. These would nonetheless require a closing cover of dirt later, so the charcoal burner might pile some extra dirt next to them in preparation, but they had to remain open to allow air in to the bottom of the pile during lighting.

Note that this was laborious, dirty work, which made charcoal burning an unpopular career, and charcoal burners were frequently ostracized from medieval society, in spite of their necessary role in the economy. They were often considered antisocial and eccentric, since they spent most of their time alone in the woods, and during those stages of the burn where they required helpers, they were frequently dirt-covered or charcoal-smeared, tired and sweaty (not conditions conducive to being social).

Lighting the pile

Once the mound was well covered, the charcoal burner would remove the chimney log (if applicable). In a separate, small hearth he would build a normal fire to generate hot coals. These hot coals he would shovel up and transfer into the open chimney of the mound. Once the seasoned wood caught fire, ideally starting at the bottom of the hole, it would be fed oxygen by the air vents—allowing the charcoal burner to control the air intake on the fire. The fire at the center of the mound had to reach 350ºC to start pyrolysis (3) or charring. Thick smoke, white from steam, would issue from the top of the central chimney hole; the whole pile might issue steam.

Smothering & shrinkage

As moisture content dropped, the smoke would turn from dense and white to blue-tinted, or even clear (2). This process might take hours or even days. When the charcoal burner observed this change in smoke, he would then cover the vents and the chimney hole, smothering it with dirt and stopping the aerobic combustion of the wood. The mound had to be observed closely during this phase, to make sure that no cracks formed and no red heat was visible through the dirt. If any cracks formed in the dirt sealing the mound, then air would get in to reignite the fire, wasting fuel by burning it. The charcoal burner had to watch closely and patch every crack as soon as possible. The wood would continue to smolder, sometimes for days or weeks before it cooled.

Making and maintaining such a mound required a rotation of unskilled laborers (often teenage boys). The physical size of the mound would shrink by a third, as moisture escaped and burnt material turned to ash. The dirt cover had to be tamped down on the top of the shrinking pile, preventing pockets of open space that might allow woodgas to build up. Woodgas is explosive—if it accumulated and became hot enough to ignite, the pile would blow up. Thus, the helpers had to climb onto the hot mound and stamp the dirt down to compact it. If the dirt collapsed unexpectedly, they could fall into the hot charcoal (which might be around 1,000ºC at this point). Sometimes they wore ropes, so that they might be pulled from the mound and survive, though burned very badly.

Yield

It could take many days, or even weeks, for the charcoal-and-earth pile to cool enough to safely open it. Opening it when enough heat remained for combustion would simply ignite the pile, consuming the whole batch of charcoal and wasting the hard work that went into making it. When fully cooled, the dirt would be raked off the top and sides, trying to keep as much dirt off the valuable product underneath as possible. The charcoal would then be ready to dig out, break into lumps, sort by quality and size, and transport away to market, monastery, city, kiln or forge—and the charcoal burner could start to prepare his next pile.

Yield varied based on the quality of the wood used (was it oak? was it dried to about 15% moisture content? was it mostly sap wood or heart wood, and were the log rounds all equal diameter?) and the skill and dedication of the charcoal burner who controlled the process. A highly skilled charcoal burner, who could ensure even gas circulation inside of the mound and time the smothering and opening correctly, could expect as much as a 25% yield in charcoal (weight of charcoal produced compared to weight of cordwood used). However, data from third world charcoal production today suggest that a16% yield (1 ton of charcoal for every 6 tons of fuelwood) would have been the more typical result of the mound method (2).

As should be evident, producing charcoal this way was very labor intensive and required at least one dedicated, year-round worker per site to oversee the whole operation. Still, most of the labor of a burn could be supplied by unskilled workers in shifts, who could return to their normal jobs as soon as the burn was complete. The tools and infrastructure involved were all quite basic (axes, shovels and rakes) and the resulting product was so dramatically superior that it was considered well worth the labor invested.

From a modern perspective, the mound method seems wasteful and dangerous. It releases woodsmoke to pollute the air, and requires large quantities of wood to be cut down—not to mention the labor hours invested in it. However, it is important to look at the process in context and compare it with other fuels available, both then and today. In a well-built mound, most of the particles and woodgas from smoke are actually consumed (as they are in a modern high-efficiency woodstove) and contribute their extra energy toward heating the fuelwood. Thus, the air pollution released by charcoal production is far less than that generated by many individual open-hearth fires. An enclosed, controlled burn yields less ash and smoke, and more heat, than an open fire, where ambient heat loss and exposure to drafts both decrease the efficiency of the conversion of fuel to heat.

Regarding labor, many medieval foresters and charcoal burners maximized their output per hour by maintaining coppices of fuel species, harvesting the poles at the ideal diameter so that they never wasted effort in splitting wood to the proper size for charcoal production. The use of charcoal also saved labor on the domestic side of the equation, because the consumer who burned charcoal in a brazier spent less time splitting kindling or fuel wood, and got more heat energy per kg of fuel they carried in, and had more control over the speed and timing of their burns (banking back the coals for overnight slow burns, or feeding more air and fuel for hotter fires when cooking). The consumer also had less cleaning to do with charcoal use than with firewood (carrying firewood into a home and storing it beside a hearth is inherently messy, with bark and mud and bugs often shedding onto the floor) and did not need to build a chimney into their dwelling.

Finally, the safety of using charcoal, compared with firewood burning, meant less waste from structure fires and the resultant losses in property, goods and lives. Also, the lack of smoke from burning charcoal meant reductions in smog accumulation over cities, and prevented ailments caused by urban smoke-inhalation.

Since the use of coppices for growing the renewable resource of fuel wood to make charcoal guaranteed an efficient and environmentally-friendly supply that was constantly available, one might argue that the use of charcoal was actually cleaner and more sustainable than most modern energy sources used today. Indeed, many developing nations still rely upon charcoal for their energy needs, and there is growing interest in modern biochar production as a renewable and clean energy source for the developed world. However, the mound method has long since been proven as one of the less efficient methods for creating charcoal, even if it was simple and low-investment. It has long since been replaced in commercial operations by “retorts” and industrial kilns.

To read about another primitive method of charcoal production, see Pit Kilns.  To read about other uses for charcoal, see Micro-Charring.  To read about wood preservation, see Charring Fence Posts.

Sources:

1) Groenewoudt, Bert and Spek, Theo. “Woodland Dynamics as a Result of Settlement Relocation on Pleistocene Sandy Soils in the Netherlands (200 BC-AD 1400).” Rural Landscapes: Society, Environment, History, Vol. 3(1), p.1, 2016.

2) “Simple Technologies for Charcoal Making.” Food and Agriculture Organization of the United Nations, FAO Forestry, Paper 41, 1983.

3) Warren, Graeme; McDermott, Conor; O'Donnell, Lorna and Sands, Rob. “Recent Excavations of Charcoal Production Platforms in the Glendalough Valley, Co. Wicklow.” NUI, The Journal of Irish Archaeology, 21 : 85-112, 2012.